1. Field of Invention
The present invention relates to a group III-V semiconductor substrate. More particularly, the present invention relates to a group III nitride vertical-rods substrate.
2. Description of Related Art
Currently, light emitted diodes and laser diodes are widely applied. For example, the combination of the blue emitting device made of gallium nitride and the yellow fluorescent powder can produce white light. The light emitted diodes and laser diodes not only can provide lights with higher brightness than those provided by the traditional light bulbs but also consume less power than that consumed by the traditional light bulbs. Moreover, the life time of the light emitted diode is about ten-thousand hours which is longer than that of the traditional light bulb.
The light emitted diodes producing colorful light including red, green, blue and ultraviolet, in the market, are mainly made of gallium nitride compound series. Since the lattice constant, the thermal expansion coefficient and the chemical properties of the sapphire are different from those of the gallium nitride, the gallium nitride growing on the heterogeneous substrate, such as silicon substrate, silicon carbide substrate and sapphire, possesses many defects and dislocations. Those dislocations will cause the increasing of the loss of emitted light by gallium nitride based light emitting devices. This kind of dislocation affects the performances and the life times of the light emitted diode and the gallium-nitride-series laser diode.
In order to decrease the numbers of the threading dislocations, several substrate structures are developed in conventional technology. FIG. 1 is a cross-sectional view showing a conventional group-III nitride substrate. As shown in FIG. 1, a substrate 100 has a gallium nitride buffer layer formed thereon and there are several barrier patterns 104 disposed on the gallium nitride buffer layer 102. A semiconductor layer 106, which is the gallium nitride epitaxial layer, grows from the exposed gallium nitride buffer layer 102 between the barrier patterns 104 and covers the barrier patterns 104. This kind of substrate structure utilizes barrier patterns to block partial of the dislocations so that the portion of the gallium nitride epitaxial layer grown over the barrier patterns 104 does not possess threading dislocations. However, the gallium nitride still has serious local dislocation phenomenon. On the other words, the portion of the gallium nitride not over the barrier patterns has relatively higher distribution of the dislocations.
FIG. 2 is a cross-sectional view showing a conventional group-III nitride substrate. As shown in FIG. 2, a substrate 200 has a buffer layer 202 and a crystal seed layer 204 formed thereon. Then, several trenches 206 are formed to penetrate the buffer layer 202 and the crystal seed layer 204. That is, the buffer layer 202 and the crystal seed layer 204 are patterned to be the strip-like structures. By using the selectively lateral growth of the heterogeneous substrate, which is so-called pendeo-epitaxy (PE), the gallium nitride only suspensorily-and-laterally grows at the sidewall of the strip-like crystal seed layer 204 and then covers the strip-like crystal seed layer 204 for keeping the gallium nitride epitaxial layer from growing vertical penetrating dislocations. Similar to the gallium nitride epitaxial layer growing on the substrate structure shown in FIG. 1, the aforementioned suspensory growth of the gallium nitride epitaxial layer possesses local threading dislocation problem. On the other words, the possibility for forming the threading dislocation phenomenon highly is concentrated at some local area. Therefore, the gallium nitride epitaxial layer growing on this kind of substrate structure is not the total dislocation free.
Because both of the group-III nitride epitaxial layers growing on the aforementioned substrate substrates have threading dislocation problems, the thickness of the group-III nitride epitaxial layer is limited to the dislocation phenomenon and is less than 20 micron meters.